A metallized film capacitor is generally used by putting a capacitor element in a case to prevent invasion of surrounding humidity, and armoring by pouring various potting resins. Roles of potting resin and case are retention and insulation of contained capacitor element and components, and prevention of deterioration of characteristics. It is hence necessary to select materials excellent in mechanical strength, low water absorption, heat resistance, heat cycle resistance, and electrical characteristics (insulating performance). Potting resins include epoxy resin and urethane resin.
Recently, in inverter system for driving motor of hybrid electric vehicle (HEV), film capacitors excellent in dielectric strength, low loss and temperature characteristics are gradually replacing the existing aluminum electrolytic capacitors. Since car-mount capacitors are used in cold district, and hot and damp district, as higher heat cycle resistance in a wide range of temperature and humidity is demanded than in consumer electric appliances. It is also required to have a structure small in heat generation if a high ripple current flows, or a structure having low inductance characteristics in order to suppress generation of surge voltage.
FIG. 7 is a sectional view of a conventional film capacitor. In an ordinary structure of film capacitor, a metal terminal 23 connected to an electrode 22 of a film capacitor element 21 is projected from a potting resin 24. In this case, when a heat cycle from low temperature (for example, −50 to −40° C.) to high temperature (for example, 90 to 120° C.) is applied, a thermal stress is generated due to difference in coefficient of thermal expansion between the potting resin 24 and metal terminal 23, and the potting resin 24 may be cracked it not withstanding the stress.
To overcome such problem, it has been proposed to prevent crack by improving armoring resin materials.
For example, as disclosed in Japanese Utility Model Application Laid-Open No. H5-18020 (document 1), Japanese Patent Application Laid-Open No. S56-101731 (document 2), and Japanese Patent Application Laid-Open No. S54-63300 (document 3), by using two or more layers of resin as armoring material of film capacitor element, it is proposed to improve the environmental resistance, mechanical strength, and explosion-proof property. In these proposals, the resin of first layer directly covering the film capacitor element is made of a urethane resin of smaller strength and elasticity than epoxy resin, and the second layer at the case opening side is made of epoxy resin.
For example, document 3 does not relate directly to film capacitor, but suggests improvement of heat cycle resistance, by blending inorganic filler by 40 to 95 vol. %, so that the difference in coefficient of linear expansion between the conductor or electric device and hardened matter of thermosetting resin for covering them directly may be 15 ppm/K or less.
However, the prior arts involve some problems as discussed below. In the structure of document 1 or document 2 in which urethane resin is disposed around the film capacitor and an epoxy resin is disposed on the upper layer, if exposed to temperature of −40° C., the epoxy resin layer of surface is cracked.
In the car-mount film capacitor, a low inductance characteristic is demanded in order to realize low exothermic property and suppress surge voltage. To satisfy these requirements, the bus bar electrode terminal connected to the film capacitor element is broadened in width (increased in sectional area), so that low resistance and low inductance are realized. However, in the structures disclosed in document 1 and document 2, when the bus bar electrode buried in the resin is broadened, it is found that cracks are likely to be formed in the armor resin. In particular, at ambient temperature of −40 to 100° C., cracks are formed in all samples in heat cycle test.
When the film capacitor element is covered with a soft urethane resin, the element size increases, and when expansion or shrinkage amount increases, motion of film capacitor element cannot be restrained, and repetitive stresses occur in the electrode portion of film capacitor element, thereby increasing tan δ. Or when the film capacitor element is covered with urethane, moisture is absorbed easily, and humidity resistance is not expected. Or as in document 3, when the amount of inorganic filler is increased so as to decrease the coefficient of linear expansion of hardened matter, liquid viscosity when pouring the resin (60° C.) becomes very high, 5000 to 20000 mPa·s. If such resin is poured in a large amount, voids are likely to occur in the resin, and pouring time must be extended, and other problems occur in working efficiency and productivity.
When the technology of document 3 is applied in film capacitor, difference in coefficient of linear expansion from metal electrode material is decreased, and cracks of resin upon heat cycle may be prevented, but difference in coefficient of linear expansion is increased, to the contrary, from the film capacitor element having the coefficient of linear expansion about one digit higher, and capacitor characteristics are likely to deteriorate in long-term heat cycle durability.